If engines with a compresion ratio 9:1 and under run fine off regular gas, and the Taco V6 has a compresion ration of 10:1 that leaves it right on the border. because of this i have been puting premium in my truck. But is this overkill? I don't want to use 87 octane, but since my engine is just barely above the magic number, would it be more efficiant using 89 as opposed to 91 or 93? I'm wondering if the highest octane is not being utilized by my engine and robbing it of power and mileage? I know the higher the octane the higher the compression ratio needs to be to get the maximum combustion from the fuel. Is 10:1 high enough to fully take advantage of 91 or 93, or do i need to drop a level? i would appreciate any feedback from anyone that knows anything about compression and octane levels. Thanks in advance

1GR-FE 4.0L V6 from a 2007 Toyota Tacoma. The 1GR-FE is the 4.0L version. Bore is 94 mm and stroke is 95 mm. Output is 236 hp (183 kW) at 5200 rpm with 266 lb-ft (382 Nm) of torque at 4000 rpm on 87 octane, and 239 hp at 5200 rpm with 278 lb-ft at 3700 rpm on 91 octane. This engine features Toyota's single VVT-i, variable valve timing, system and a compression ratio of 10.0:1. Inside, the 1GR uses a taper-squish combustion chamber design with matching pistons to improve anti-knocking and engine performance, while also improving intake and fuel efficiency. Toyota adopted a siamese-type intake port, which reduces the surface area of the port walls and prevents fuel from adhering to such walls. This engine has special cast-iron cylinder liners cast into the block, which are a spiny type to improve adhesion between the liner and cylinder block. With these special thin liners it is impossible to bore the block. In the event of cylinder wall damage (scoring, deep protrusions, etc), the entire cylinder block must be replaced. For increased block rigidity, the 1GR also receives a high temperature plastic insulator/protector, which fills the empty space between the outer portion of the cylinders and block material common to open deck engines. For increased cooling efficiency, the 1GR employs water passages between the bores of the engine. There are such 2 passages for each bank for a total of 4. This reduces cylinder hot-spotting and keeps combustion chamber temperatures more uniform.

I've experimented with both 87/89/91 octane. My experience has been that running 89 over 87 the mileage has improved 1-2 mpg,(that in itself pays for the 10 cents higher cost over 87) there's also less downshifting and slightly lower cruise rpm's. I've ran different brands of gas, drove the same routes to and from and consistently found this to be the case. However, I have found absolutely no improvement between running 91+ over 89. Therefore, based on the 10 to 1 compression ratio it appears logical that a 89 octane would outperform an 87.